The present invention relates generally to a hydraulic actuator and, more particularly, to an improved hydraulic actuator for use in an active mount for a vibrating component in a system for reducing vibration and noise transmission from the vibrating component to a support structure.
Hydraulic actuators are used in numerous environments to induce movement of one object with respect to another object. A hydraulic actuator generally includes a cylinder and a moveable piston inside the cylinder. A piston rod is connected to the piston and extends outwardly from one end of the cylinder where the rod end is attached to the first object. The other end of the cylinder is mounted, directly or indirectly, to the second object. The means for mounting the piston rod and cylinder to the objects may incorporate flexible bearing assemblies to provide some xe2x80x9csoftnessxe2x80x9d to the attachment. Such bearing assemblies preferably comprise elastomeric material. Pressurized hydraulic fluid is introduced into the interior of the cylinder on one or both sides of the piston to effect longitudinal movement of the piston in the cylinder so that the objects are moved relative to one another.
Hydraulic actuators may be used as a component of an active mount in a system for reducing vibration and noise transmission from a vibrating component to a support structure. For example, hydraulic actuation systems are used for actively reducing the vibratory and acoustic loads on aircraft, particularly rotary wing aircraft such as helicopters. A primary source of vibratory and acoustic loads in a helicopter is the main rotor system. The main rotor system of a helicopter includes rotor blades mounted on a vertical shaft that projects from a transmission, often referred to as a gearbox. The gearbox comprises a number of gears which reduce the rotational speed of the helicopter""s engine to the much slower rotational speed of the main rotor blades. The gearbox has a plurality of mounting xe2x80x9cfeetxe2x80x9d which are connected directly to structure in the airframe which supports the gearbox. The main rotor lift and driving torque produce reaction forces and moments on the gearbox. All of the lift and maneuvering loads are passed from the main rotor blades to the airframe through the mechanical connection between the gearbox feet and the airframe. The airframe structure which supports the gearbox is designed to react to these primary flight loads and safely and efficiently transmit the flight loads to the airframe.
In addition to the nearly static primary flight loads, the aircraft is also subjected to vibratory loads originating from the main rotor blades and acoustic loads generated by clashing of the main rotor transmission gears. The vibratory and acoustic loads produce vibrations and audible noise that are communicated directly to the helicopter airframe via the mechanical connection between the gearbox and the airframe. This mechanical connection thus becomes the xe2x80x9centry pointxe2x80x9d for the unwanted vibration and noise energy into the helicopter cabin. The vibrations and noise within the aircraft cabin cause discomfort to the passengers and crew. In addition, low frequency rotor vibrations are a primary cause of maintenance problems in helicopters.
Active vibration and noise reduction systems in aircraft utilize sensors to monitor the status of the aircraft, or the vibration producing component, and a computer-based controller to command actuators to reduce the vibration and noise. The sensors are located throughout the aircraft and provide signals to the adaptive controller. The controller provides signals to the hydraulic actuation system, including a plurality of actuators located at strategic places within the aircraft. The actuators produce controlled forces or displacements which attempt to minimize vibration and noise at the sensed locations.
Two methods of actuator placement are frequently used in the active system: (1) distribution of actuators over the airframe, or (2) co-location of the actuators at, or near, the vibration or noise entry point. When applied to the main rotor system of a helicopter, the co-location approach places the actuators at or near the structural interface between the transmission and airframe stopping vibration and noise near the entry point before it is able to spread out into the aircraft. This has the advantage of reducing the number of actuators and the complexity of the control system. Active systems using this approach to counteract vibration and noise employ actuators mounted in parallel with the entry point or in series between the transmission gearbox feet and airframe support structure.
When the actuator is mounted in series with the vibrating component and the support structure, the point of attachment of the piston rod has six possible degrees of freedom. However, only the degree of freedom along the principle load carrying axis can be actively controlled for vibration and noise reduction. Further, since the elastomeric bearing is located between the piston rod and the attachment point, the bearing assembly must provide high static and dynamic stiffness along this active, load carrying axis so that motions of the piston translate directly into unattenuated motions at the attachment point. To ensure that motions at the attachment point along the five non-active degrees of freedom do not create vibration and noise, the stiffness between the attachment point and actuator along these directions must be low. However, the need for the elastomeric bearing to be stiff along its principle load carrying axis yet soft about the other five degrees of freedom can cause the elastomeric bearing to be unstable under load. Also, transverse motions and rotational forces on the piston can induce high loads between the piston and cylinder which may cause the piston to bind.
Moreover, since hydraulic actuators operate under high pressure, the leakage of hydraulic fluid often occurs, particularly where the piston rod passes through the end of the cylinder. This leads to maintenance problems as well as environmental concerns. Additionally, escaping hydraulic fluid can damage the elastomeric material of the bearing assembly. Cylinder seals are usually provided to prevent leakage of hydraulic fluid from the cylinder. Cylinder seals are typically flat, disc-shaped bodies having a central opening. The seals peripherally engage the cylinder body at an outer edge and receive the piston rod through the central opening. The cylinder seal allows movement of the rod relative to the cylinder while at the same time sealing the interface between the rod and cylinder to block fluid leakage along the rod. However, cylinder seals have been known to wear out frequently, primarily due to friction between the rod and seal which results in deterioration of the sealing function and eventual leakage. When the seal fails, replacement requires the removal of the entire hydraulic actuator from service.
For the foregoing reasons, there is a need for an active mount including a hydraulic actuator comprising an elastomeric bearing which is stable under load. The new hydraulic actuator should withstand the significant loads generated when used in an active mount for the transmission of the main rotor system of a helicopter. Further, any transverse or rotational forces on the actuator should not induce high loads between the piston and the housing. The new hydraulic actuator should also have improved means for preventing leakage of hydraulic fluid from the cylinder. The leakage preventing means should protect the elastomeric components of the bearing assembly from damage by hydraulic fluid. The leakage prevention means should also have excellent wear characteristics.
Therefore, it is an object of the present invention to provide a hydraulic actuator including an elastomeric bearing that remains stable under load and does not induce high loads between the piston and cylinder due to transverse motions and rotations at the point of attachment.
It is also an object of the present invention to provide a hydraulic actuator which prevents leaked hydraulic fluid from contacting the elastomeric bearing assembly.
According to the present invention, a seal assembly is provided for use in a hydraulic actuator of the type including a cylinder, a piston movably disposed in the cylinder, and a piston rod connected to the piston and extending from an end of the cylinder. The seal assembly comprises a seal member formed of a material impervious to hydraulic fluid and having an inner edge and an outer peripheral edge. The inner edge of the seal member defines a central opening which is larger than the outside diameter of the piston rod for receiving the rod. Means are provided for sealing the inner and outer edges of the seal member adjacent the end of the cylinder so that the inner edge of the seal member is radially spaced from the outside diameter of the piston rod and the seal member forms a fluid impervious wall. A bearing assembly may be connected to the end of the piston rod externally of the actuator cylinder and the sealing means may comprise first and second rings removably attached to the bearing assembly so that the inner and outer edges, respectively, of the seal member are sealingly secured between the rings and the bearing assembly for preventing hydraulic fluid from contacting the elastomeric portion of the bearing assembly. Preferably, the seal member material is flexible for accommodating relative movement between the piston and the cylinder.
Also according to the present invention, an active mount is provided for mounting a vibrating component to a support structure, such as a gearbox to the airframe of an aircraft, for reducing vibration transmission from the vibrating component to the support structure. The active mount comprises a housing adapted to be attached to one of the vibrating component or the support structure and a hydraulic actuator disposed in the housing. The hydraulic actuator includes a cylinder, a piston movably disposed in the cylinder, a piston rod connected to the piston and extending outwardly of an end of the cylinder and connected to the other of the vibrating component or the support structure. A seal member formed of a material impervious to hydraulic fluid and having a central opening which is larger than the outside diameter of the rod for receiving the rod is sealed in the cylinder so that the inner edge of the seal member is radially spaced from the outside diameter of the piston rod and the seal member forms a fluid impervious wall. A bearing assembly may be connected between the second end of the piston rod and the other of the vibrating component or the support structure and the sealing means may comprise first and second rings removably attached to the bearing assembly so that the inner and outer edges, respectively, of the seal member are sealingly secured between the rings and the bearing assembly for preventing hydraulic fluid from contacting the bearing assembly. Preferably, the seal member material is flexible for accommodating relative movement between the piston and the cylinder.
According to another aspect of the present invention, a hydraulic actuator is provided for an active vibration and noise control system in an aircraft, the actuator including a mounting member for attaching to the vibrating component and a bearing assembly engaged with mounting member and the piston. The bearing assembly comprises an inner bearing member, a first bearing located between the mounting member and the inner bearing member for transmitting axial motions between the mounting member and the piston, a second bearing located between the inner bearing member and the housing for transmitting moment and shear loads from the mounting member to the housing, and a third bearing located between the inner bearing member and the piston for permitting rotational movement of the inner bearing member relative to the piston. The bearings of the bearing assembly are located so that their central axes are substantially aligned.
The coincidence of the central axes of rotation of the several bearings provides a bearing assembly which is stable under load. This arrangement also significantly reduces the force which is transmitted between the piston and housing when the elastomeric bearing is loaded, which must be kept low to ensure the piston does not bind in its bore. The coincidence of the centers of rotation allows the use of a spherical bearing as the third bearing with a very low rotational stiffness. Hence rotation of the radial journal bearing relative to the spherical bearing creates low torques between the two which in turn creates low reaction forces between the piston and housing. The sealing assembly prevents leaked hydraulic fluid from reaching, and damaging, the elastomeric bearing.
The foregoing and other features and advantages of the present invention will become more apparent in light of the following detailed description of the preferred embodiments thereof, as illustrated in the accompanying figures. As will be realized, the invention is capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and the description are to be regarded as illustrative in nature, and not as restrictive.